To assess the reliability of M-mode echocardiographic patterns of mitral valve prolapse (MVP) (echo MVP) in detection of morphologic evidence of MVP (morphologic MVP), operatively excised mitral valves and corresponding M-mode echocardiograms from 65 patients with chronic, severe, isolated, pure mitral regurgitation (MR) were studied. Of the 65 patients, 45 (69%) had echo MVP (either holosystolic or mid-to-late systolic prolapse patterns on preoperative M-mode echocardiogram) and 42 (93%) of them had morphologic MVP; of the 3 without morphologic MVP, 2 had ruptured chordae tendineae from infective endocarditis and 1 had papillary muscle dysfunction from atherosclerotic coronary heart disease. Of the 20 patients without echo MVP, 14 (70%) had no morphologic MVP (9 had papillary muscle dysfunction from coronary heart disease, 4 had infective endocarditis on previous normal valves and 1 had rheumatic heart disease). Of the 48 patients with morphologic MVP, 42 (88%) had echo MVP and most had considerably dilated mitral anulae; the other 6 had ruptured chordae tendineae with less degrees of anular dilatation. Of the 17 patients without morphologic MVP, 3 had echo MVP (coronary artery disease in 1 and infective endocarditis on a previous normal valve in 2); of the 14 with neither echo nor morphologic MVP, 9 had papillary muscle dysfunction from coronary artery disease, 4 had infective endocarditis on previously normal valves and 1 had rheumatic heart disease. The patients with very dilated mitral anuli and leaflet areas generally had holosystolic (hammocking) patterns on echo; the patients with small anuli and leaflet areas usually had mid-to-late systolic (buckling) prolapse patterns.